1. Field of Invention
This invention relates to a method and system for optical single-sideband data generation and transmission.
2. Description of Related Art
The ability of the Mach-Zehnder (MZ) intensity modulator to control both intensity and optical frequency has been demonstrated to improve the performance of long-haul fiber-optic systems such as those used by the telecommunications industry. See, F. Koyama and K. Iga, xe2x80x9cFrequency chirping in external modulators,xe2x80x9d Journal of Lighwave Technology, Vol. 6, pp. 87-93, 1988. In addition, the use of high-power diode-pumped YAG lasers operating at 1300 nano-meters (nm) and MZ external intensity modulators based on LiNbO3 has found wide application in the cable TV (CATV) industry.
Conventionally, optical transmission systems may use either direct or external modulation of a laser. Ideally, the intensity of the light output from a modulated laser should be linearly proportional to the injected current, and the frequency of the optical carrier should be minimally influenced. However, modulation of the optical frequency occurs when a laser is directly modulated. For example, a data signal may be added directly to the laser current, so that the laser output, i.e., the optical carrier, is intensity modulated. In theory the bandwidth of the transmitted signal must be as broad as the bandwidth of the data signal. However, the resultant optical spectrum is more than twice as broad as the theoretical limit. Not only does the optical signal have double-sidebands, one below and one above the optical carrier, the intensity modulation also superimposes an unintentional frequency modulation, i.e., laser frequency chirp. Laser frequency chirp is modulation of the laser frequency caused by modulation of the refractive index of the laser cavity in response to current modulation
The interaction of chirp and chromatic dispersion in the fiber can cause system impairments. Therefore, to avoid the effects of laser frequency chirp, externally-modulated optical fiber links are conventionally recognized as the preferred choice. By using an MZ external modulator it is possible to modulate an optical carrier so that the resulting optical spectrum does not have any excess chirp. See, id. F. Koyama and K. Iga, xe2x80x9cFrequency chirping in external modulatorsxe2x80x9d. However, the optical spectrum will still have double-sidebands, and therefore, be twice as wide as the theoretical limit.
FIG. 1 shows schematics of radio frequency (RF) and optical spectra to illustrate these points. FIG. 1 shows the data signal used to modulate the optical carrier, in this example the data signal is made of multiple RF subcarriers (CATV signals are a good example of this sort of signal). FIG. 2 is a schematic of the spectrum of an externally modulated optical carrier. Note that it is twice as wide as the spectrum shown in FIG. 1.
Conventional intensity modulation creates signals with two sidebands around the optical carrier frequency. These two sidebands contain the same information. Because of optical fiber dispersion, different frequency components will travel at different speeds, creating interference in the transmitted signals. Although the two sidebands contain the same information, they travel at different speeds in the optical fiber and arrive at the receiver at different times. The net result is a power penalty and limit in the transmission distance. The greater the frequency separation the higher the penalty.
Optical transmission systems employing baseband digital transmission, e.g., by on/off keying of the light, may also suffer from the effects of dispersion. In long-distance transmission systems, dispersion can interact with non-linearities in the optical fiber, further impairing transmission.
All conventional fiber optic communication systems employ double-sideband modulation. To reduce the effects of dispersion it is preferable to either operate at wavelengths corresponding to low-fiber dispersion, or include dispersion compensation. Some optical fibers also suffer from polarization-mode-dispersion, which may vary with time due to strain and temperature variations. It is difficult to compensate for this sort of dispersion. In addition, some optical non-linearities, such as self-phase modulation, are worse in transmission systems with low dispersion. Therefore; there is a need for optical modulation techniques that are tolerant of dispersion.
FIG. 4 shows a conventional fiber optic data transmission system 400. In the system 400, an optical carrier signal is emitted from optical source 410. The carrier signal is modulated by optical modulator 412, which is driven by a modulating signal 415, to generate an optical signal consisting of an optical carrier signal with double-sideband, DSB 420. However, when the DSB signal 420 is sent over fiber link 425, chromatic dispersion causes each spectral component to experience a different time delay depending on the length of the fiber link 425. The transmitted DSB signal 420 is received by a photodetector 435 coupled to the fiber link 425. This photodetector 435 converts the incident optical DSB signal 425 into current. The photodetector 435 generates a current corresponding to the received optical power Pr which has a direct current part corresponding the average received optical power and an alternating current part which corresponds to the instantaneous optical intensity change due to modulation.
However, if the phase difference between the two optical sidebands at optical frequencies (fcarrier+fRF) and (fcarrierxe2x88x92fRF) received at the photodetector 435 is an odd multiple of xcfx80, the received signal from the upper sideband and the lower sideband will destructively interfere with each other, canceling out all the information power in the signal received by the photodetector 435 at fRF. As the frequency fRF increases, the dispersion effect causes impairments at shorter transmission distances. As a result, the length of the fiber link 425 becomes severely limited. For example, when conventional single-mode fiber is employed, a 3-dB degradation in the detected RF power occurs in an externally modulated, 6 km link operating at 1.5 xcexcm with a 20 GHz sub-carrier.
Therefore, chromatic dispersion can be a major factor limiting the maximum distance and/or bit rate of long haul fiber-optic systems that require relatively lengthy optical links. Dispersion compensation can mitigate these effects, but it adds to the system""s complexity.
FIG. 3 shows an optical signal with single-sideband transmission. The transmission of single-sideband (SSB) signals has also been used in RF transmission systems to reduce the RF electromagnetic spectrum occupied by the signal by a factor of two. The use of optical SSB transmission also reduces the transmitted optical signal spectrum by a factor of two. The smaller the bandwidth used in transmission, the smaller the dispersion penalty in the transmitted signal. Therefore, because only half of the optical bandwidth is required, the dispersion suffered by an optical single-sideband signal is half of the same signal using double-sideband modulation. In an intensity modulated double-sideband optical transmission system, the detected signal is generated by mixing the two sidebands with the optical carrier in the transmitted spectrum shown in FIG. 2. The down-converted signal has components from both the upper and lower sidebands.
However, the relative delay between different corresponding frequency components in the upper sideband and the lower sideband are different, although they represent the same information, making it difficult to compensate for optical fiber dispersion in the electrical domain. In contrast, in an optical single-sideband transmission system, the detected baseband signal is generated in the photodetector by mixing the optical carrier signal with only one sideband. Therefore, the relative arrival times of the various signal frequency components are preserved in the electrical output signal resulting from photodetection. As a result, the dispersion effect due to the transmission fiber link can be compensated in the electrical domain after photodetection. Such compensation is advantageous because electrical compensation can be done using microwave delay lines which are much more compact than dispersion compensation fiber.
The most obvious method for generating optical SSB signals is to use an optical filter to suppress one of the sidebands. However, this method is limited by the characteristics of optical filtersxe2x80x94currently available optical filters are not sufficiently sharp to be used to generate single-sideband signals when the modulating signal has low-frequency content.
Alternatively, it is also known to generate an optical transmission signal with SSB modulation rather than filtering out one of the sidebands. See, M. Sieben, J. Conradi, D. Dodds, B. Davies and S. Walklin, xe2x80x9c10 Gbit/s optical single sideband systemxe2x80x9d, Electronics Letters, May 22, 1997, pp. 971-3. In such a scheme, light from a laser is modulated using an MZ intensity modulator, with the two sides of the modulator driven in such a way as to create a single-sideband, intensity-modulated output. Thus, if m(t) is the input data, and H(m(t)) is an approximation of the Hilbert transform of m(t), then one side of the modulator is driven with m(t)+H(m(t)xe2x88x92Vxcfx80/4, and the other side is driven with xe2x88x92m(t)+H(m(t))+Vxcfx80/4, where Vxcfx80 is the voltage required to induce a xcfx80 phase shift to the optical signal in each arm of the MZ interferometer. To a first order, the signal generated by such a modulation system is a good approximation of the single-sideband optical signal.
However, although this alternative scheme is more practical because no optical filtering is required, it does not work well for low-frequency information, owing to the imperfection of the response of a practical Hilbert transformer in the low-frequency region. As a result, the lower-frequency components of the optical signal still have sidebands on both sides of the optical carrier signal.
The present invention solves the deficiencies of the conventionally known SSB systems and methods for generating and transmitting optical SSB signals by line-encoding the input data before generating the optical SSB signal. Byline coding the input data, the low-frequency portion of the transmitted signal is removed, reducing the non-ideal effects of a practical Hilbert Transformer.
Such a technique improves a fiber-optic system""s dispersion tolerance, including polarization-mode dispersion. In addition, such an improvement in spectral efficiency leads to increased fiber-optic system capacity and longer transmission distance.
These and other features and advantages of this invention are described in, or are apparent from, the following description of the apparatus/systems and methods according to this invention.